Prevention of corrosion in alkali metal halide solutions by ammonia addition



\ 3,354,062 PREVENTION OF CORROSION IN ALKALI METAL HALIDE Nov. 21, 1967 L M. DVORACEK SOLUTIONS BY AMMONIA ADDITION Filed Sept. 21, 1964 2 Sheets-Sheet 1 INVENTOR. z. M ax 04 4651? Nov. 21, 1967 SOLUTIONS BY AMMONIA ADDITION 2 Sheets-Sheet Filed Sept. 21, 1964 WXARNO N .llllnll Q A a 6 J 5 M 2 H & 5 H I 0 l llll'l INVENTOR United States Patent 3,354,062 PREVENTIGN 0F (ZGRROSION IN ALKALI METAL HALHDE SULUTKQNS BY AMMONIA ADDITl0N Louis M. Dvoracek, Bren, Caiif assignor to Union Gil Company of (laliforuia, Los Angeles, (Ialitl, a corporation of California Filed Sept. 21, 1%4, Ser. No. 397,710 11 Claims. (Ci. 204-147) This invention relates to a method of preventing corrosion of ferrous metals in contact with aqueous alkali metal halide solutions, and comprises passivating the metal surfaces exposed to corrosive aqueous alkali metal halide solutions with critical amounts of ammonia.

The various alkali metal halides have found wide usage in industry. Aqueous solutions of these salts are used as heat transfer media, both in refrigeration and heating systems, and as raw materials, intermediate products and treating agents in the chemical process industry. Because of their clarity, high density, and low cost, certain of these salt solutions are particularly desirable oil well packer fluids, in which application a concentrated salt solution is injected into a well bore above a mechanical packer to maintain a static fluid head on the upper surface of the packer, thereby counter-balancing formation pressure below the packer.

Although highly useful in a diversity of industrial ap plications, aqueous solutions of the alkali metal halides will corrode carbon and commercial alloy steels, particularly at elevated temperatures. The corrosion of storage tanks, piping, oil well tubing and casing, and process equipment by aqueous solutions of these salts may be prevented by utilization of stainless steel, or by glass or plastic liners installed in the equipment. However, these expedients are costly, or lack the durability of carbon steel construction. Carbon and commercial alloy steel may be used in aqueous alkali metal halide service if a suflicient external electric current is provided to aflord cathodic protection, thereby reducing or eliminating corrosion. Techniques of achieving cathodic protection are well known; the necessary electric current being provided from an external electric source or from sacrificial anodes placed in the corrodent media. Although usually effective in providing protection for metal surfaces exposed to a corrodent, cathodic protection systems are generally relatively costly to install and maintain.

The principal object of my invention is to provide a simple, low cost means of preventing corrosion of carbon and commercial alloy steels exposed to aqueous alkali metal halide solutions.

A more specific object is to provide a method of preventing corrosion of oil well tubing and liners exposed to aqueous alkali metal halide packer fluids.

Other objects will be apparent to those skilled in the art from the more detailed description of my invention.

I have now found that these objects can be achieved by addition of a critical amount of ammonia to the corrosive alkali metal halide solution to thereby effectively passivate the metal surface. The critical quantity of ammonia necessary to achieve passivity is dependent on the specific metal to be protected, the specific alkali metal halide and its concentration, and the temperature. In any case, the necessary quantity of ammonia is limited; insuflicient and excessive amounts both resulting in corrosion, as will be more fully explained. The ammonia addition may also be combined with either chemical or electrical treatment to achieve passivation of the metal surfaces at ammonia concentrations incapable of otherwise effecting passivation.

Corrodents to which my passivation technique is applicable are aqueous solutions of the alkali metal halides ice including the chloride, bromide, iodide and fluoride salts of sodium, potassium, lithium, rubidium, and cesium, such as sodium chloride, sodium bromide, sodium iodide, sodium fluoride, potassium chloride, potassium bromide, potassium iodide, potassium fluoride, lithium chloride, lithium bromide, lithium iodide, lithium fluoride, rubidium chloride, rubidium bromide, rubidium iodide, rubidium fluoride, cesium chloride, cesium bromide, cesium iodide, and cesium fluoride. The corrodent may be an aqueous solution of a pure salt, mixtures of these salts, or aqueous solutions of one or more of the above salts combined with other extraneous materials.

I have found that ferrous metals such as carbon and commercial alloy steels, i.e., those steels containing less than about 4% chromium and less than about 6% nickel with or without other elements such as vanadium, molybdenum, tungsten, etc., are amenable to passivation in aqueous solutions of alkali metal halides by the addition of ammonia to the corrosive solution. Accordingly, as used herein, ferrous metals that can be passivated with ammonia in accordance with my invention comprise steels having chromium contents from about 0 to about 4%, nickel contents from about 0 to about 6%, and not more than about 4% of other alloying elements. Illustrative of metals protectable by ammonia passivation are the common carbon and commercial alloy steels such as AISI Types 1010, 1020, and 4130 and API Grades 1-55, N- and P-110.

My invention will now be described by reference to the drawings of which:

FIGURE 1 illustrates an anodic polarization curve for a typical passivated carbon steel.

FIGURE 2 illustrates potential-time curves I and metal loss curves II for a stable active state (a) and a stable passive state (b).

The anodic polarization curve of FIGURE 1 represents the current-potential relationship of a typical steel in a system that exhibits passivity. The anodic polarization curve is constructed by potentiostatic techniques, wherein a metal test electrode is immersed in the corrosive solution contained in an electrolytic cell. A controlled potential is applied to the electrode and the resultant electrical current observed. The applied potential is varied and the current observed at a series of test points to obtain data from which the polarization curve can be constructed.

Referring now to the polarization curve of FIGURE 1, the segment of the curve between A and B is the active state wherein corrosion occurs by electrochemical dissolution of the metal. Point B is identified as the maximum positive critical anodic current inflection point, also known in the art as the critical anodic current peak, or Flade point. The maximum' positive current inflection point of the anodic polarization curve is defined as the point of inflection of the polarization curve at positive anodic current densities where the anodic current density reaches a maximum value and where any change in the potential, either in the electronegative orelectropositive direction, causes a decrease in anodic current density. The current density at the maximum positive critical anodic current inflection point is the critical anodic current density and its corresponding potential is the pria mary passive potential. Many systems demonstrate a plurality of positive anodic current inflection points, such as B, which have lower maximum current densities than primary peak B. I have generally observed that systems characterizedby low critical anodic current densities, say less than about +20 microamperes/sq. cm., are transient in that the critical anodic current densities decrease with time, usually to negative current values. Stable passivity cannot be achieved under conditions whereby the current inflection points are transient. In such cases, the system must be subjected to additional treatment to effect stability.

The region of the anodic polarization curve between points C and F is the passive state. The continuous'line through points C, D, E, and F represents a system which exhibits stable passivity and wherein little or no corrosion occurs. Such'stable passivity is achieved only if the anodic polarization curve also exhibits an inflection point in the negative current region at potentials more positive than the primary passive potential and a null point of zero current density at a potential more positive than the potential at the maximum negative current inflection point, i.e., at potentials more positive than the primary passive potential, the anodic polarization curve must pass through a region of negative current density and, at still higher potential, then increase to a zero or slightly positive current density. It maybe seen from FIGURE 1 that the continuous line passes into a negative current region between points C and D, reaches a maximum negative current at D, the maximum negative current inflection point, and then increases to zero current again at point E.

The requirements for stability of a metal in a passive, non-corroding state in a solution are therefore that: (1) the anodic polarization curve of the metal exhibit a positive current inflection point, (2) at potentials more positive than that corresponding to the positive current infietcion point, the curve also exhibit a negative current inflection point and (3) at still higher potential, the anodic polarization curve exhibit a null point of zero current density. The curve in FIGURE 1 exhibits these inflection points and point E, the null point of zero current density, therefore represents a stable passive condition and the potential associated therewith, a stable passive potential. In all systems, typified by FIGURE 1, which exhibit stable passivity, the electrostatic potential of the metal when contacted with the solution will equilibrate from any potential between B and G to the potential coincident with zero current which is the stable passive potential represented by point E. As used herein, the stable passive potential is defined as that electrostatic potential atwhich the anodic current density is zero in a system which is characterized by an anodic polarization curve that exhibits a maximum positive anodic current inflection point and, at an electrostatic potential between the potential at the positive anodic current inflection point and the potential at the null current point, also exhibits a maximum negative anodic current inflection point.

The region between points F and G is the transpassive state wherein corrosion can occur or oxygen can be evolved. The broken line CE illustrates a condition wherein the anodic polarization curve does not pass below zero in the passive state, thus the passive state is not stable. A self-sustaining passive film can only be formed when the anodic polarization curve contains a maximum negative current inflection point, or negative current loop, such as that illustratad by the curved segment through points C, D and E of the polarization curve of FIGURE 1. Neither passivation, nor a stable active state can be attained unless the polarization curve passes through a maximum positive current inflection point, or maximum current peak such as illustrated at point B, but the system potentials will remain transient, that is in a condition of constant change; The stability of the active and passive states can be determined from potential-time and metal loss measurements, and may be predicted from the characteristic features of the anodic polarization curve.

FIGURE 2 illustrates typical potential-time and metal loss curves for the stable active and stable passive conditions. Curve Ia is the potential-time curve for a stable active state. Curve Ila is the cumulative metal loss curve associated with potential-time curve Ia. As may be seen from these two curves, the potential does not change with time, and, after an initialperiod of stabilization, the corrosion rate is fairly constant.

Potential-time curve Ib and metal loss curve Ilb illustrate a stable passive condition. When the passive state is stable, the corrosion potential increases, that is becomes more electropositive as it approaches zero, until a stable potential, corresponding to point B of FIGURE 1, is reached. Thereafter, the potential remains constant with time. correspondingly, corrosion takes place until a stable potential is reached, at which point no additional metal loss occurs. A metal surface which is passivated, resulting in a condition of no potential change or corrosion with time, and essentially no current flow between the metal surface and the corrodent solution, is considered as being in the stable passive state. The electrochemical potential associated with this condition is regarded as a stable passive potential, as previously described with reference to the anodic polarization curve. A stable passive potential must be reached for the metal to be effectively passivated against corrosion attack. Most of the metals, which may be passivated by the addition of ammonia according to the method of my invention, demonstrate stable passive potentials between about 200 millivolts and about -600 millivolts with reference to a standard saturated Calomel electrode.

The amount of ammonia required to passivate a metal surface exposed to a saturated alkali metal halide solution generally faiis between about 2 and about 20- weight percent, and is a function of the specific corrodent, its concentration, temperature and metal type. The stress condition of the metal also affects the requisite amount, the required ammonia concentration increasing as the stress is increased. For any set values of the aforementioned conditions, self-passivation, that is passivation of the metal by the addition of ammonia alone, without further treatment, is generally achieved over a rather narrow concentration range, not usually having more than two percentage points difference between the minimum and maximum limits of the range. This narrow critical range required for self-passivation usually falls between the limits of about 2 and about 10 weight percent ammonia. At too low ammonia concentrations the metal will corrode. At ammonia contents within the critical concentration range, a self-sustaining stable passive film affording substantial corrosion protection will be formed and no further treatment will be necessary'so long as the ammonia concentration is maintained. At ammonia contents above the critical range for self-passivation, an active state will result and corrosion will continue at fairly constant rates; Therefore, to effect self-passivation of the metal, it is essential that sufficient ammonia be initially added to achieve the proper concentration of ammonia in the solution, and that this concentration be maintained during the period that the metal is exposed to the corrodent.

The limits of the critical composition range for selfpassivation can be established under actual operating conditions, or under controlled test conditions simulating actual conditions. The effect of various ammonia concentrations may be detected by potential-time and metal loss determinations, or such effect may be predicted from the anodic polarization curve.

The pH of the corrodent solution is a critical factor in passivation of a metal surface, even though passivation, as practiced herein, cannot be achieved by merely adding non-ammoniacal basic substances to the solution. For instance, in the case of carbon or commercial alloy steels immersed in saturated sodium chloride solution, I have found that the pH of the solution must be increased to at, least about 10 coincident with ammonia addition to effect passivation of the metal, since pH values below about 10 are not conducive to the formation'of a stable passive film.

As previously mentioned, the invention is based on my discovery that metal surfaces in contact with a corrosive aqueous alkali metal salt solution can be passivated by adding ammonia directly to the corrodent salt solution. The ammonia can be added as anhydrous or aqua ammonia, as as an ammonium salt, such as those of the halides. The aforementioned amount of ammonia, 2 to weight percent for self-passivation, are sufiicient to raise the pH of alkali metal salt solutions to above about 10 when aqua or anhydrous ammonia are used. When an ammonium salt is used as the source of ammonium ion, suflicient alkaline metal hydroxide, for example sodium or potassium hydroxide, should also be added to increase the solution pH to a value above the critical. The addition of ammonia to an alkali metal halide solution in the aforementioned quantities, i.e., to provide an ammonia concentration from about 2 to 10 weight percent and adjustment of the pH of the solution to at least about 10, will impart the previously described inflection and null current points to the anodic polarization curve.

This invention is generally applicable to all systems wherein one or more of the aforementioned metals is in.

contact with an aqueous alkali metal halide solution including generally all temperatures encountered therein, e.g., temperatures from the freezing point of the solution to about 750 F., or higher, and with suflicient pressures at said temperature to maintain the solution in liquid phase and to maintain said ammonia concentration. Pressures to which this passivation technique is adapted can vary from subatmospheric to 10,000 p.s.i.g., or more.

As indicated above, the ammonia concentration must be within critical limits to afford stable se'lf-passivation. However, where the ammonia content of a corrosive alkali metal halide solution is outside the critical concentration range, passivation may nevertheless be achieved by the addition of a strong oxidizing agent, such as hydrogen peroxide, ozone, sodium chromate, sodium dichromate, or potassium permanganate, to the solution. These oxidizing agents cannot produce passivity by themselves. However, when used in combination with the addition of ammonia at concentrations below the critical range, the oxidizing agent functions to stabilize the passive state at a lower ammonia concentration than would other :ise

be obtainable. Similarly, under conditions of too high ammonia concentration, the oxidizing agent functions to convert the stable active state to a stable passive state. Generally, passivaion of a ferrous metal by the combination of ammonia and oxidant addition may be effected in alkali metal halide solutions containing between about 2 and about 20 weight percent ammonia. The amount of oxidizing agent necessary in any particular application is dependent on the ammonia concentration, metal type, stress, the specific corrodent, its concentration, and temperature. In general, between about zero and about one weight percent of the oxidizing agent added with ammonia Will'achieve stable passivity. However, in some circumstances, additional oxidizing agent up to as high as about 5 percent may be required.

Corrosion of a ferrous metal in an aqueous alkali metal halide solution containing ammonia can also be arrested by application of an external electric current, even though the ammonia concentration is outside the critical concentration range for self-passivation. Where the ammonia content of the corrosive solution is in excess of that required to achieve self-passivation, a corrosive stable active state will be formed. This stable active state may be converted to a non-corrosive stable passive state by the application of anodic current sufficient to increase the electrostatic potentialof the system to a value more positive than the potential at the maximum positive current inflection pointQOnce stable passivity is achieved, the anodic treatment may be discontinued and the metal will remain passivated. Generally, ferrous metal-aqueous alkali metal halide systems containing between about 2 and about 20 weight percent ammonia can be converted from the stable active state to the stable passive state by anodic treatment.

In the. case of low'ammonia concentrations, that is concentrations below the critical range for self-passivation and usually from about zero to about 6 weight percent ammonia, cathodic current is required. Even though insuflicient amomnia is added to produce a self-passivating stable passive film, ammonia addition will substantially reduce the amount of current necessary for cathodic protection. Thus, ammonia addition at concentrations up to about 6 weight percent amomnia can effect economy in a cathodically protected system, even though the ammonia concentration by itself would be insuflicient to yield stable passivity. This reduced current requirement is attributable to the passivation effect of the amomnia, and not to the deposit of a calcareous, or other, scale on the metal surface.

My invention will now be illustrated by the following examples:

EXAMPLE I The anodic polarization curve of type 1020 carbon steel in an aqueous sodium chloride solution was determined by means of an automatic, electronic potentiostat. The potentiostat is a device capable of measuring the current flowing between electrodes immersed in an electrolyte solution at a controlled electrostatic potential. Both current and potential can be accurately measured, the potential being measured relative to a standard electrode, such as a saturated Calomel electrode. A test electrode was machine from type 1020 carbon steel, cleaned by polishing with number 600 grit abrasive paper, rinsed with 10 percent hydrochloric acid and then immersed in a saturated sodium chloride solution contained in the polarization cell of the potentiostat. The temperature of the polarization cell was controlled at 75 F. and its contents were continuously stirred during the test. No maximum inflection points in the anodic polarization curve were observed.

The corrosion rate of type 1020 carbon steel in saturated sodium chloride solution was determined at 75 F. by means of a standard corrosometer. Measurements were taken over a 120 hour period. Corrosion was found to be fairly uniform during this period, amounting to a total of 530 microinches, corresponding to a corrosion rate of 0.039 inch/year.

Ammonia was added to a saturated sodium chloride solution to obtain a series of solutions having contents of 0.75, 1.5, 3.0, 3.8 and 4.5 weight percent amomnia. Anodic polarization and corrosion rate determinations were made for type 1020 carbon steel in these solutions at 75 F. At the lower ammonia concentrations of 0.75 and 1.5 weight percent, maximum positive current inflection points were detected, but these points were transient and the metal did not exhibit passivity. Observed maximum current densities were less than +20 microamperes/sq. cm. at potentials between 800 and 825 millivolts. At the 3.0 and 3.8 weight percent ammonia concentrations, maximum positive current inflection points at +70 and microamperes/sq. cm. were observed, at primary passive potentials of +840 and 860 millivolts, respectively. Both polarization curves contained negative current loops, i.e., had maximum negative current inflection points.

Determinations of the potential-time and corrosiontime relationships were made on type 1020 carbon steel immersed in saturated sodium chloride solutions containing 3.0, 3.8 and 4.5 weight percent ammonia, respectively. In the solution containing 3.0 weight percent ammonia, corrosion occurred for an initial period of about 30 hours, and then ceased. During this period the potential increased from about 920 millivolts to about 240 millivolts, at which point the metal surface was passivated. The potential between the metal and the solution remained constant at about 240 millivolts for the balance of the hour test period. With the 3.8 weight percent solution, corrosion occurred until the potential increased from an initial value of about +1020 millivolts to a stable passive potential of about 470 millivolts. Thus, it was established that type 1020 carbon steel could be passivated from sodium chloride attack at 75 F. by the addition of 3.0 and 3.8 weight percent of ammonia.

Similar potential-time and corrosion-time determinations. at 4.5 weight percent ammonia concentration indicated the formation of a stable active state with corrosion continuing for the duration of the test period. The potential was observed to remain at about 930 millivolts, indicating that the addition of 4.5 weight percent ammonia to the saturated sodium chloride solution resulted in the formation of a stable active state.

EXAMPLE II An anodic polarization curve was developed by the methodof Example I for type 1020 carbon steel in an aqueous solution saturated with sodium fluoride and containing 4.5 weight percent ammonia at a temperature of 75 F. A maximum positive current inflection point was noted at a current density of +45 microa'mperes/ sq. cm. and a primary passive potential of -880 millivolts. A negative current loop was also noted indicating the formation of a stable passive state.

EXAMPLE VII The passivation of various metals in saturated sodium chloride solutions were determined by anodic potentiostatic analyses according to the method described above. Results of these determinations are as follows:

MAXIMUM POSITIVE CURRENT INFLEC'IION POINT Ammonia Current Primary Maximum Negative Test Metal Concentration, Temperature, Density, ,ua./ Passive Current Infiection Passive Passive State weight percent F. sq. cm. Potential, Point Formed Potential Stable millivolts AISI Type 304. 4. 75 None None None No. AISI Type 4130. 4. 5 200 +55 -1,060 540 Yes. API Grade L55. 4. 5 200 920 400 No 6.0 200 +505 1,040 480 Yes. API Grade N-80. 4. 5 200 +490 1,050 500 Yes. API Grade P-110 4. 5 200 +10 880 360 N o.

6.0 200 +275 920 340 Yes.

EXAMPLE 111 Example I was repeated using an aqueous solution saturated with potassium iodide and containing 4.5 Weight percent ammonia. A maximum positive current inflection point was noted at a current density of microamperes/ sq. cm. and a primary passive potential of -860 millivolts. The passive state was stable.

EXAMPLE IV The experiment of Example I was repeated using a saturated solution of potassium bromide with 4.5 weight percent amm'onia added. A maximum positive current inflection point of +130 microamperes/sq. cm. was observed at a primary passive potential of 880 millivolts. A negative current loop indicated the formation of a stable passive state under test conditions.

EXAMPLE V This example demonstrates the passivation of carbon steel from corrosion by aqueous sodium chloride solution at elevated temperatures. Anodic polarization, potentialtime and corrosion rate data were obtained for carbon steel in aqueous saturated sodium chloride solutions at 180 F. with ammonia additions of 1.5, 2.3, 3.0, 3.8 and 4.5 weight percent.

A maximum positive current inflection point was not observed at 1.5 Weight percent ammonia. At 2.3, 3.0 and 3.8 weight percent ammonia addition, maximum positive current inflection points were observed, but were transient. At 4.5 weight percent ammonia, however, the initial active state was converted to -a stable passive state at a potential of -350 millivolts, with no corrosion.

The results obtained in this example and in Example I indicate that carbon steel in aqueous solutions of sodium chloride can be passivated from corrosion by the addition of ammonia, the required ammonia concentration increasing with temperature. For example, passivation was achieved at 3.0 to 3.8 weight percent ammonia at 75 F., Whereas 4.5 weight percent was required at 180 F.

EXAMPLE VI A- method of stabilizing the passive state by the addition of sodium chromate was demonstrated. The addition 1 Neither active nor passive states were obtained with the type 304 stainless steel. However, each of the carbon and commercial alloy steels tested were successfully passivated by the addition of proper amounts of ammonia. The passive state of API Grades J-55 and P-'-110 were not stable at 4.5 weight percent ammonia, but these steels were successfully passivated at 6.0 weight percent. The instability of the passive state at the lower ammonia concentrations for these steels was caused by the transient nature of the maximum positive current inflection point which had current densities of +10 microamperes/sq. cm. in both cases. These values were below the minimum required for stability, which is about +20 microamperes/ sq. cm.

EXAMPLE VIII The effect of pH on the passivation of carbonsteel was determined by potentiostatic polarization studies of acarbon steel electrode immersed in an aqueous solution ofsodium chloride containing ammonium chloride equivalent to an ammonia addition of 4.1 weight percent and having a pH of 5.2. No passivity was observed.

The solution pH was increased by adding ammonium hydroxide and sodium hydroxide in proportions sufficient to maintain the ammonium concentration constant. Stable passivity was observed when the solution pH reached 10.2. Thus, it was demonstrated that passivity cannot be achieved by ammonia addition alone, unless a pHof at least about 10 is also achieved. 7

The effect of pH on the passiv'ation of carbon steel in aqueous solutions of sodium chloride containing no ammonia was determined by potentiostatic analyses of sodium chloride solutions containing various concentrations of sodium hydroxide. A stable passive state was not produced at pH values up to 14, thus demonstrating the beneficial effect of ammonia in passivating ferrous metals from-alkali metal halide corrosion. I

Various other changes and modifications of the inven tion are apparent from the description thereof and further modifications and variations will be obvious to those skilled in the art. Such modifications and changes are intended to be included within the scope of'this invention as defined by the tollowing claims:

I claim:

1. The method of preventing corrosion of ferrous metals in contact with a corrosive aqueous alkali metal halide solution which comprises: adding ammonia to said aqueous solution to provide therein an ammonia concentration between about 2 and about 20 weight percent ammonia and to produce a stable passive electrostatic potential between said metal and said solution.

2. In a system comprising a corrodable ferrous metal in contact with a corrosive aqueous alkali metal halide solution, the method of arresting the corrosion of said metal which comprises: adding ammonia to said solution to provide therein an ammonia concentration between about 2. and about 20 weight percent ammonia and to impart to the anodic polarization curve of said metal in said solution (1) a maximum positive current inflection point, (2) a maximum negative current inflection point at an electrostatic potential more positive than the potential at said maximum positive current inflection point, and (3) a null point of zero current density at a stable passive potential more positive than said potential at said maximum negative current inflection point.

3. The method of claim 2 wherein said ammonia added to said solution is selected from the group consisting of aqua ammonia, anhydrous ammonia and a mixture consisting essentially of an ammonium salt and an alkali metal hydroxide.

4. The method of claim 2 wherein said alkali metal is selected from the group consisting of sodium chloride, potassium chloride, sodium bromide and potassium bromide.

5. The method of claim 2 wherein said ammonia is added to said solution to provide therein an ammonia concentration between about 2 and about 10 weight percent ammonia, to impart to said solution a pH above about 10, and to impart to said anodic polarization curve, a current density at said maximum positive current inflection point exceeding :20 microamperes/sq. cm.

6. The method of claim 2 wherein an oxidizing agent selected from the group consisting of sodium chromate, sodium dichromate, potassium permanganate, hydrogen peroxide and ozone is added to said solution to provide therein a concentration of oxidizing agent not exceeding about 5 weight percent of said oxidizing agent.

7. In the method of claim 2 wherein said solution containing said added ammonia has a pH above about 10, and wherein said anodic polarization curve has a current density at said maximum positive current inflection point above about :20 microamperes/sq. cm., the additional steps comprising:

passivating said metal by impressing an anodic current on said metal in said solution sufficient to produce a more positive electrostatic potential between said metal and said solution than the potential at said maximum positive current inflection point of said anodic polarization curve.

8. In a system wherein a cathodic electric current is applied to a corrodable ferrous metal in contact with a corrosive solution consisting essentially of Water and alkali metal halide to reduce the rate of corrosion of said metal in said corrosive solution, the improvement which comprises: adding ammonia to said aqueous solution to reduce the amount of cathodic current required to arrest said corrosion.

9. The system of claim 8 wherein said ammonia added to said solution is selected from the group consisting of aqua ammonia, anhydrous ammonia and a mixture consisting essentially of an ammonium salt and an alkali metal hydroxide.

10. The method of passivating a ferrous metal surface exposed to a corrosive aqueous alkali metal halide solution which comprises:

determining the electrostatic potential between said metal and said solution with reference to a standard saturated Calomel electrode; adding ammonia to said aqueous solution to reduce said potential to a stable passive potential between about -200 millivolts and about 600 millivolts relative to said saturated Calomel electrode; and

maintaining the concentration of said ammonia in said solution at a level suflicient to sustain said stable passive potential.

11. The method of claim 10 wherein said ammonia added to said solution is selected from the group consisting of aqua ammonia, anhydrous ammonia and a mixture consisting essentially of an ammonium salt and an alkali metal hydroxide.

References Cited UNITED STATES PATENTS 1,395,730 11/1921 Reichinstein 21-2.7 2,343,085 2/1944 Savell 212.7 2,474,603 9/1949 Viles et a1. 212.7 2,582,138 1/1952 Lane et a1. 212.7 2,755,166 7/1956 Marsh 21-2.7 3,074,862 1/ 1963 Sudrabin 204148 JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,354,062 November 21, 1967 Louis M. Dvoracek It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 9, lines 38 and 49, for each occurrence, read Signed and sealed this 3rd day of December 1968.

(SEAL) Attest:

Edward M. Fletcher, J r. EDWARD J. BRENNER Commissioner of Patents Attesting Officer 

1. THE METHOD OF PREVENTING CORROSION OF FERROUS METALS IN CONTACT WITH A CORROSIVE AQUEOUS ALKALI METAL HALIDE SOLUTION WHICH COMPRISES: ADDING AMMONIA TO SAID AQUEOUS SOLUTION TO PROVIDE THEREIN AN AMMONIA CONCENTRATION BETWEEN ABOUT 2 AND ABOUT 20 WEIGHT PERCENT AMMONIA AND TO PRODUCE A STABLE PASSIVE ELECTRROSTATIC POTENTIAL BETWEEN SAID METAL AND SAID SOLUTION. 